1. Field of the Invention
The present invention relates to an automatic correction apparatus for a video signal of a digital VTR, which is suitable for structuring a dubbing system by connecting digital VTR's in cascade to avoid quantization distortion even if there is variation in the level or the frequency response of an analog transmission path.
2. Description of the Prior Art
In the case of structuring a dubbing system using digital VTR's, it is usual that the VTR's are connected with an analog video signal path, as shown in FIG. 3.
FIG. 3 will be explained below. In FIG. 3, 1 and 2 designate digital VTR's for a reproducer and a recorder, respectively. These are connected by an analog transmission path 3. The reproducer 1 comprises a magnetic head 4, a reproducing digital processing circuit 5, a digital-to-analog converter 6 and an analog output processing circuit 7, to reproduce a video signal recorded on a magnetic tape 8.
The recorder 2 comprises an analog input processing circuit 9, an analog-to-digital converter 10, a recording digital processing circuit 11 and a magnetic head 12, for recording a video signal on a magnetic tape 13.
The analog output processing circuit 7 and the analog input processing circuit 9 have adjustable resistors 14 and 15, respectively, to adjust these circuits to an optimum state and to absorb attenuation of a signal in the transmission path 3.
The existing quantization level of a digital VTR has 8 bits, and particularly the D2VTR (refer to SMPTE 247M and 244M of the SMPTE Journal, July 1990) has allocated the levels of a video portion into 140 steps of quantization from the black level to the 100% white level.
For example, assume the case of dubbing the 100% ramp signal as shown in FIG. 4 in an analog transmission path by using the D2VTR system. As shown in FIG. 4, 8-bit digital codes (200) and (60) correspond to the 100% white level and the black level respectively in the 8-bit digital codes (0-255) of the D2VTR system.
FIGS. 5 and 6 show changes in waveforms of a ramp signal at the time of dubbing.
In FIG. 5, oblique line 16 shows a slope of an original ramp signal at the normal level. Circle marks a to l on the oblique line 16 show picture elements of the signal. It is assumed here that the quantization level increases by one step per one picture element. Oblique line 17 shows a slightly lower setting of levels of the recorder 2 in the analog-to-digital converter 10 in the dubbing system in FIG. 3, with the largest error occurring at a sample point l corresponding a point l , deviated by a 1/2 step. However, deviations up to a 1/2 step become the same waveform as that of the original ramp signal as shown in sample points a to l on an oblique line 18, after the analog-to-digital converter 10.
FIG. 6 shows a case of a fairly lower setting of levels of the recorder 2 in the analog-to-digital converter 10, with errors exceeding a 1/2 step in sample points a to l on the oblique line 19, and an oblique line 20 shows an input waveform in the analog-to-digital converter 10. In this case, after the analog-to-digital converter 10, the signal has a stage in the middle as shown by an oblique line 21. When a ramp signal (the oblique line 21) obtained is dubbed by the same dubbing system in the same condition, a signal with a larger stage is obtained as shown in an oblique line 23.
From the above, it is understood that a large quantization distortion occurs when the adjustment of the analog circuit is deviated in the dubbing system in FIG. 3.
It is also understood from the above that it is necessary to minimize the level error to within a 1/2 step in order to prevent a quantization distortion from occurring and that it is necessary to minimize the gain deviation to within 1/280 (0.4%) in the dubbing system in order to prevent a quantization distortion from occurring in the 140 steps between the black level and the 100% white level of the ramp signal shown in FIG. 4.
It is almost impossible in practice to minimize the gain deviation to these values partly because of uncertain attenuation in the transmission path 3.